1. Field of the Invention
The present invention relates to a wireless local loop (WLL) system, more particularly relates to a call processing method, a subscriber unit, and an access control apparatus in an access network (AN) in a wireless local loop (WLL) system.
The wireless local loop (WLL) system is the system being most watched in the different types of access mode systems at the present and can be considered one of the leading means for connecting a local exchange (LE) and subscriber unit (SU) with a radio section interposed between them.
There are two leading means for concretely constructing such a wireless local loop (WLL) system: The first is to construct a wireless local loop (WLL) system by using a cellular network for mobile telephones as a base, while the second is to construct a wireless local loop (WLL) system by interposing a so-called "personal handy-phone" (PHS) system network. The wireless local loop (WLL) system referred to in the present invention may be any of the above structures. In this case, from the viewpoint that the first wireless local loop (WLL) system (cellular network) requires a mobile station controller (MSC) and therefore the compatibility with a public switched telephone network (PSTN) is not so good (explained later) and so on, the second wireless local loop (WLL) system (PHS network) is more preferred. Namely, when this second wireless local loop (WLL) system is used, it is easy to harmonize with the above-mentioned public switched telephone network (PSTN) and form a more broadly defined public switched telephone network (PSTN).
In such a more broadly defined public switched telephone network (PSTN), however, it is necessary to introduce an inter-network interface for connecting an access network (AN) including a subscriber unit (SU) and the public switched telephone network (PSTN) containing the local exchange (LE). As a concrete example of this inter-network interface, as will be mentioned later, there can be mentioned &lt;1&gt; the typical analog interface of a 2W (pair wire) type for accommodating general telephone (analog) signals and &lt;2&gt; a V5.2 (analog) interface according to V5.2 of the ITU-T recommendation. The explanation of the present invention given later is made by taking the latter V5.2 interface as an example.
2. Description of the Related Art
The wireless local loop (WLL) system to which the present invention is applied will be explained later in further detail by referring to the drawings. This wireless local loop (WLL) system forms a more broadly defined public switched telephone network (PSTN) by harmonizing with already existing public switched telephone networks (PSTN). Further, in this more broadly defined public switched telephone network (PSTN), an access network (AN) containing the wireless local loop (WLL) system is formed.
This access network (AN) contains, for example, a base transceiver station (BTS) connected to a subscriber unit (SU) provided with a user terminal (UT) such as a telephone set via a radio section and a base station controller (BSC) connected to the base transceiver station (BTS) via an access network interface (I/F).
In order to realize the above more broadly defined public switched telephone network (PSTN) as a practical network sufficiently satisfying user needs, there are a large number of problems which have to be solved. It is important to mainly solve four problems (1) to (4) among them. A detailed description will be made of these four problems (1) to (4) later. Simply mentioned, however, they are as follows.
Problem (1): When channel congestion etc. occur, it becomes impossible to even dial a number. This is a serious problem if the originating call is an emergency one etc.
Problem (2): Even if pre-connected channels are prepared, "very important phone (VIP) calls" of the V5.2 interface cannot be always guaranteed to get through.
Problem (3): At the time of channel congestion etc., not only when the originating call is a general call, but also when it is a "very important phone" (VIP) call, call origination from a user terminal (UT) is sometimes rejected.
Problem (4): Even if an assignment succeeds and a free time slot (TS) is provided for a user terminal (UT), if the user fails to capture a radio channel in the radio section in the wireless local loop (WLL) system, he will fail in connection in the radio section not only when the call is a general call, but also when it is a "very important phone" (VIP) call.